In Code Division Multiple Access (CDMA) communication systems, multiple data channels are provided by spreading the data of individual users with unique spread codes. Traditionally, the elements that are communicated on the data channels are referred to as ‘symbols’ and the elements of the spread codes are referred to as ‘chips’. An example of such a CDMA signal is the Wideband Code Division Multiple Access (WCDMA) signal specified by the 3rd Generation Partnership Project (3GPP) standards organization. Other examples for mobile wireless networks are the CDMA2000 family of signals and the TD-SCDMA signals contained in these two alternative CDMA technologies. These standardized CDMA technologies provide third generation (3G) mobile voice/phone and internet/data service to a growing number of wireless subscribers/users around the world.
At the CDMA signal transmitter, a CDMA signal for multiple channel transmission can be created by summing different spread code channel signals. These individual code channel signals are created by modulating a selected spread code sequence with a symbol from an assigned user data channel. At the CDMA signal receiver, knowledge of the spread code used at the transmitter is required so that the receiver can extract the data/symbol of interest to the user. This code knowledge is provided to the user's receiver, for example, as part of the network-subscriber link acquisition procedure. Under ideal radio wave propagation conditions, the performance of the CDMA receiver for an individual user data channel does not benefit from knowing the spread codes that are simultaneously being used on channels that have been assigned to other users.
Under realistic, non-ideal radio wave propagation conditions and in the real world environment of multiple base station networks, the multiple user/multiple base station CDMA signals interfere with each other such that the performance of traditional, ‘assigned code only’ receivers, can be severely degraded. An example of an ‘assigned code only’ receiver is a code-matched, channel-matched filter receiver which is also known as the implementation of a ‘Rake’ receiver which is well known in the art. When used for the downlink receiver in a subscriber handset, the Rake receiver displays performance degradation with increasing levels of intracellular and/or intercellular interference.
Intracellular interference refers to the multiple user interference within a single-base-station cell that arises due to multiple propagation path (multipath) distortion of the radio signal. Multipath distortion causes the spread codes associated with multiple code channels to loose their mathematical property of orthogonality. This loss of spread code orthogonality due to multipath results in a performance degradation of the legacy ‘assigned code only’ Rake receiver. This performance degradation is sufficiently severe to make the use of the Rake receiver undesirable for 3G networks designed to provide mobile wireless broadband service to multiple users. An ‘equalizer receiver’ is based on a signal-estimation filter that approximately corrects the multipath distortion and approximately restores the orthogonality property of the multiple spread code signals that are contained in the received CDMA signal. An ‘assigned code only’ despreading operation will then provide a high performance detection of the symbols that are of interest to the user, even in propagation environments that contain significant multipath-derived intracellular interference.
Intercellular interference refers to the multiple base station interference that arises whenever the subscriber's received signal contains signals originating from two or more base stations transmitting on the same radio frequency. The topology and the frequency usage of the 3G CDMA networks results in intracellular interference being a significant factor in a large amount of the cell coverage area. Both the legacy Rake receiver and the equalizer receiver are sensitive to intercellular interference and incur significant performance degradation when it is present.
More complicated, ‘advanced receivers’, that can mitigate the effects of intercellular interference for real world multiple user/multiple base station CDMA networks, include techniques that address intercellular interference such as ‘interference suppression by means of projection’, for example U. Madhow and M. L. Honig, MMSE Interference Suppression for Direct-Sequence Spread-Spectrum CDMA, IEEE Transactions on Communications, Vol. 42, No. 12, pp. 3178-3188, December 1994, and techniques that perform interference cancellation where interfering signals are estimated and subtracted. For examples of the latter type of advanced receiver see A. Bastug and D. Slock, Interference Canceling Receivers with Global MMSE—Zero Forcing Structure and Local MMSE Operations, Proc. Asilomar Conf. on Signals, Systems & Computers, November 2003.
A practical problem that arises in the implementation of interference cancellation receivers is active spread code detection, i.e., the identification of what spread codes are being used in the CDMA signals. Knowledge of the active spread codes is required to estimate the interfering CDMA signals so that they can be subtracted off. Another benefit of determining the complete set of active codes in a CDMA signal is that it allows a higher performance estimation of the propagation channel impulse response. Improving the quality of the channel estimate improves receiver performance by improving the processing that compensates for the multipath channel distortion, e.g., improved channel equalization.
Although knowledge of all active spread codes is beneficial to the implementation of advanced receivers for CDMA signals, it is standard practice for the CDMA network to give the individual user's receiver knowledge of only the spread code or codes assigned to that user. What is desired is a system and method that allows the individual user's receiver to determine the complete set of active spread codes associated with each CDMA signal that is strong enough to be received/interfere.